Mar.28 (GMM) Nico Rosberg on Saturday completed a clean-sweep of heading the practice timesheets at Albert Park, but Williams' Patrick Head remains cautious as Melbourne braces for the decisive qualifying session.

German Rosberg, 23, was three thousandths quicker than Toyota's Jarno Trulli in third and final practice in Australia, having also headed Friday's two 90 minute sessions.

"No complaints," engineering boss Head said, "it's better to be at the sharp end than the blunt end, but it's going to be very tight in qualifying.

"Very tight," he emphasised, with the other FW31 driven by Kazuki Nakajima fifth.

Like Toyota, the other 'clever diffuser' team Brawn has also been consistently quick this weekend, and early on Saturday Jenson Button was also close to the pace.

Ferrari's Felipe Massa was also up there, although teammate Kimi Raikkonen brought up the rear following an hydraulic failure.

Like Head, Brawn boss Ross Brawn was keeping his expectations for qualifying in check.

"You never know what fuel loads everyone is running, so the reality check is still to come later this afternoon.

"I'm holding back until we see what happens," he added.

Lewis Hamilton - criticised for some in the paddock on Saturday for skipping the grand prix ball on his debut as world champion - continued to struggle in practice, mid-pack and again slower than his McLaren teammate.

Sebastian Vettel has also had a scruffy opening weekend for Red Bull Racing, and on Saturday the Renaults were among the slowest cars on the track.